Image sensor arrays are used in many applications to obtain images digitally. Image sensor arrays generally comprise a plurality of discrete pixel sensors, which in some cases are arranged in columns. Each pixel sensor serves to convert a detected signal incident upon a detector to an electrical signal. In the example of a photo-detector configured to detect an electromagnetic signal, a photodiode and some form of analogue-to-digital conversion is employed. Image sensor arrays include Complementary Metal Oxide Semiconductors (CMOS) sensor arrays, Charge Coupled Device arrays (CCD), and the like.
The specification of an image sensor array, such as a CMOS image sensor array, can be described in several terms. Two terms of particular importance are dynamic range (DR) and signal-to-noise ratio (SNR). In many applications, a high dynamic range and high signal-to-noise ratio are preferred.
Linear sensors achieve a high signal-to-noise ratio, at least in part, by using photocurrent integration. However, such sensors tend to have a lower dynamic range, when compared to non-linear sensors. Time-based linear sensors have a higher dynamic range, but need a longer integration time, which limits the frame rate.
Logarithmic sensors have been shown to provide a dynamic range of over 160 dB. Such sensors generally require no integration capacitance. However, these sensors can suffer from high fixed-pattern noise and low signal-to-noise ratio. Certain calibration methods can be employed to reduce fixed pattern noise. However, the low signal-to-noise ratio continues to be a problem.
A delta-sigma analogue-to-digital converter for signal conversion can achieve higher signal-to-noise ratios, and may be used to improve the signal-to-noise ratio of image sensors, such as logarithmic image sensors.
Previous work by the inventor in this field was published in “Optimization of delta-sigma adc for column-level data conversion in cmos image sensors,” in Instrumentation and Measurement Technology Conference Proceedings, 2007 IEEE, 1-3 May 2007, pp. 1-6.
The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. One or more of the above prior-published documents or issues raised in the background may, or may not have been considered by, or known to, the inventor or have been published prior to the date of invention.